This invention relates generally to communication systems and, more particularly, to communication systems wherein information is transmitted from a main terminal serially through a plurality of remote terminals and then back to the main terminal and still more particularly to systems of such type wherein the information is time multiplexed.
Digital communication systems of the prior art are typically of the type having two terminals which communicate with each other. Each terminal includes a plurality of processing units and is adapted to transmit and receive information over a plurality of channels, each channel being associated with a corresponding processing unit. Examples of a processing unit are voice channel unit or data channel unit, referred to as a voice/data unit. In one such system, all channels transmitted by one terminal are received by the other terminal. In North America, one type of such digital carrier system is called a T1 digital carrier system having 24 channels, each associated with a corresponding one of 24 time slots. AT1 system is discussed in U.S. Pat. No. 4,059,731, issued Nov. 22, 1977 to Green et al and in U.S. Pat. No. 4,245,340 issued Jan. 13, 1981 to Landry. Other types of T1 digital carrier systems exist, such as a European 32 channel system, but in any event, both the North American and European systems operate on the principle that the plurality of channels are transmitted digitally using a serial bit stream made up of a corresponding plurality of time slots, one time slot for each channel. Thus, the data stream may be considered as being made up of a sequence of 24 time slots. It follows then that if less than 24 channels are required, the processing units associated with those unused channels remain idle. In order to improve the processing unit usage efficiency, a system was developed wherein a main terminal, in a central office, is serially coupled to a plurality of remote terminals, each one of the remote terminals having a plurality of channels with a plurality of corresponding processing units associated therewith. Thus, for example, with 4 remote terminals, each one of such terminals would include 6 processing units. Therefore, instead of having two locations, each with 24 processing units, the 24 units previously housed at one of the locations would be distributed for example, to 4 remote locations, each location having 6 processing units with the central office maintaining 24 processing units. With such arrangement, one 24 channel system at the central office can service several remote locations, each location requiring less than all 24 channels. With each remote terminal being a separate, independent terminal, determining the operational "health", that is, operational condition of each terminal, is paramount and such information should be ascertainable at the central office. Further, should any of these remote terminals fail in such a way as to threaten operation of the entire digital carrier communication system, a fail-safe protection scheme is required to ensure that the failed remote terminal does not disrupt the entire system. One such technique used to provide uninterrupted operation of the system includes apparatus for detecting a condition at a remote terminal which would normally disrupt the entire system. The apparatus then isolates the effected remote terminal. However, in such arrangement, all further "downstream" remote terminals are decoupled from the system even though such "downstream" remote terminals are otherwise operational.
As is also known in the art, each remote terminal is adapted to detect major and minor alarm conditions. These conditions are conditions which would not normally require removal of the terminal from the communication system, however, would be alarms to indicate to a person at the central office that a condition exists at the remote terminal in need of repair. With one technique, the remote terminal distorts the informational data transmitted by such terminal to the main terminal or central office. The distortion of such data, however, is undesirable since the quality of the system is, in effect, degraded.